Effects of morphine on pain-elicited and pain-suppressed behavior in CB1 knockout and wildtype mice

Abstract

Rationale: Pharmacological manipulations of the type 1 cannabinoid receptor (CB1) suggest a role for CB1 in morphine-induced antinociception, but studies utilizing CB1 knockout (KO) mice do not support this conclusion. Since studies using CB1 KO mice to study morphine's antinociceptive effects have only examined thermal nociception, this study examines these interactions in models that employ a chemical stimulus.

Objectives: To determine whether the findings obtained with thermal pain models extend to other models, the effects of morphine on acetic acid-induced writhing were examined in CB1 KO and wildtype (WT) mice. Behaviors that decrease in response to acid injection, feeding and wheel running, were also examined, and investigations were carried out in the thermal hotplate assay. The CB1 antagonist SR141716A was also examined in these assays.

Results: Morphine completely blocked acid-induced writhing (1.0-10.0 mg/kg) and increased response latencies in the hotplate (10.0-32.0 mg/kg) in both genotypes. Morphine (3.2 mg/kg) significantly attenuated the suppression of wheel running but did not completely prevent this effect in either genotype. Morphine did not alter pain-suppressed feeding. In each of these assays, morphine's effects were not altered in CB1 KO mice compared with WT mice; however, SR141716A attenuated morphine's effects in C57BL/6 mice.

Conclusions: The effects of morphine do not differ in CB1 KO and WT mice in preclinical pain models using thermal and chemical stimuli. Since SR141716A did attenuate the effects of morphine, it is possible that CB1 KO mice undergo developmental changes that mask the role of CB1 receptors in morphine's antinociceptive effects.

Fig. 1

Hotplate assay for CB1 KO and WT mice. Top, temperature-effect curves (44–56 ºC) in the absence of morphine. Abscissa: hotplate temperature (ºC). Ordinate: mean response latency in seconds. Bottom, morphine dose-effect curves (1.0 – 32.0 mg/kg) on 56º C hotplate. Abscissa: dose of morphine in milligrams per kilogram. Ordinate: percent maximum possible effect

Fig. 2

Effect of the CB1 antagonist SR141716A on morphine antinociception in the hotplate assay in C57BL/6 mice. Abscissa: treatment. Ordinate: mean response latency in seconds. Asterisk denotes statistical significance compared with saline. Double asterisk denotes statistical significance compared with morphine alone

Fig. 3

Acetic acid-induced writhing in CB1 KO and WT mice. Top, writhing in response to intraperitoneal injection of 0.56% acetic acid. Ordinate: mean number of writhes. Asterisk denotes statistical significance compared with WT (p<0.05). Bottom, effects of morphine on writhing expressed as percent inhibition of writhing. Abscissa: dose of morphine in milligrams per kilogram. Ordinate: percent inhibition of writhing. Bottom inset, effects of morphine on writhing (raw data)

Fig. 4

Effect of the CB1 antagonist SR141716A on morphine antinociception in the writhing assay in C57BL/6 mice. Abscissa: treatment. Ordinate: mean number of writhes. Asterisk denotes statistical significance compared with saline. Double asterisk denotes statistical significance compared with morphine alone

Fig. 5

Acetic acid-suppressed feeding in CB1 KO and WT mice. Top, non-suppressed feeding after saline and 1.0 mg/kg morphine. Ordinate: mean grams consumed per gram of body weight. Asterisk denotes statistical significance compared with WT (p<0.05). Bottom, effects of morphine on acetic acid-suppressed feeding expressed as percent control. Abscissa: dose of morphine in milligrams per kilogram. Ordinate: percent control consumption. Bottom inset, effects of morphine on acetic acid-suppressed feeding (raw data)

Fig. 6

Effect of the CB1 antagonist SR141716A on morphine antinociception in the feeding assay in C57BL/6 mice. Top, non-suppressed feeding after saline and 3.0 mg/kg SR141716A. Abscissa: treatment. Ordinate: mean grams consumed per gram of body weight. Asterisk denotes statistical significance compared with saline. Bottom, morphine alone and in combination with SR141716A. Abscissa: treatment. Ordinate: mean grams consumed per gram of body weight

Fig. 7

Acetic acid-suppressed wheel-running in CB1 KO and WT mice. Top, non-suppressed wheel-running after saline and 3.2 mg/kg morphine. Ordinate: mean wheel revolutions. Asterisk denotes statistical significance compared with WT (p<0.05). Bottom, effects of morphine on acetic acid-suppressed wheel-running expressed as percent control. Abscissa: dose of morphine in milligrams per kilogram. Ordinate: percent control running. Bottom inset, effects of morphine on acetic acid-suppressed wheel-running (raw data)

Fig. 8

Effect of the CB1 antagonist SR141716A on morphine antinociception in the wheel-running assay in C57BL/6 mice. Top, non-suppressed running after saline, 1.0 mg/kg morphine and 3.0 mg/kg SR141716A. Abscissa: treatment. Ordinate: mean wheel revolutions. Bottom, morphine alone and in combination with SR141716A. Abscissa: treatment. Ordinate: mean wheel revolutons. Asterisk denotes statistical significance compared with saline. Double asterisk denotes statistical significance compared with morphine alone